Blockchain Interview Questions and Answers for 2025

A must-know for anyone heading into a Blockchain interview, this question is frequently asked in interview questions for Blockchain developer.  

A hardware wallet is what stores your private keys in a USB stick-like-looking device. So far, it has been considered the most popular way to store your crypto as it does not even connect to the internet and is immune to online hackers.

This makes it highly secure and safe when it comes to the self-custody of your digital assets. E.g. Ledger wallet, trezor wallet, etc.

The current ecosystem of Blockchain technology is not at par with its web2 counterparts. The first concern of Blockchains is around scalability. There has to be a tradeoff between security, decentralization, and scale. This is popularly known as the Blockchain trilemma.

Secondly, blockchains are more of a social experiment than a tech experiment. Say even if you create a trustless system between different parties, it will still demand those parties to use it. Otherwise, it is as good as nothing.

There are also risks associated with the 51% attack. As quantum computing kicks in, there is a possibility that some nodes can gather 51% of the computational power and compromise with the system.

Although some of these concerns have been addressed through major structural updates of many chains, like the recent Ethereum merge, there is still a lot of work to be done.

Currently, over 50% of the world is connected online. This makes it a hotbed for the scammers to come up with newer ways to siphon off money from innocent folks.

This can take the shape of extortion, ransomware, identity theft, and sabotage.

The information is shared on a network between its participants. Before transmitting this information over a network, it needs to be changed into formats that can fit the standards of the channels (Every channel has a set of rules for communicating with each other).

Converting the information at both the sender and receiver end is regarded as information processing. The biggest challenge to information processing is securing it during that time. Another challenge is processing bulk information can impose a limit on performance

Blockchains can be easily explained with a simple analogous of a kitty party. A kitty party includes a group of people who come together and create a pool of funds with manageable monthly installments. One winner is chosen at random from everyone who gets the entire amount for that month. Essentially, depending on when your name is picked, you get access to a lump sum amount by depositing small amounts of cash.

Now, this system is highly transparent, and each participant in the network knows who has already received the money, who is yet to win, who all are regularly paying, etc.

If anyone decides to fraud the system, they will have to convince at least 51% of the people, if not all.

Blockchains are very similar in nature. They are distributed public ledgers where participants validate the transactions, and no single authority can make changes at will.

It's no surprise that this one pops up often in Blockchain basic interview questions.

Blockchain is a digital ledger that is managed by a group of nodes or computers operating on that Blockchain. What this means is that it is a database that is not owned by a single central authority.

A transaction can be entered into the Blockchain only if 51% of the total nodes agree on it.

Since these nodes can be set up anywhere in the world (distributed), it is virtually impossible to influence them for a malicious attack against the network.

On top of that, since no single authority can make independent decisions, blockchain is an immutable source of data storage, i.e., data cannot be changed once stored.

This makes Blockchains a powerful tool to run and manage a system where two transacting parties lack trust.

A common question in Blockchain interview questions, don't miss this one.  

As mentioned earlier, for a transaction to enter into the Blockchain, at least 51% of the nodes should agree on it. But then, how do these nodes agree with each other? This is through something called a consensus mechanism. While there are multiple consensus mechanisms out there, let me explain through the example of Bitcoin, which uses a proof-of-work consensus mechanism.

In a proof-of-work consensus mechanism, it is transmitted to all the nodes whenever a transaction is done. Each of these nodes has a copy of the Blockchain and hence tries to check if the transaction that is happening is valid or not.

To prove the validity of this transaction, these nodes solve a complicated mathematical problem. The first node to solve this problem is rewarded with Bitcoin (in the case of Bitcoin Blockchain), while others simply confirm whether the solution is correct.

Once the consensus is reached, this transaction is appended to the block, and this block joins the previous blocks.

Some of the most popular Blockchain platforms are as follows:

Trustlessness can be defined as the state where the two transacting parties do not have to trust each other before getting into a deal. For example, usually, this trust is based on personal experience. Whenever the parties interact at scale, this trust is often outsourced to a third party, like banks or insurance companies.

However, Blockchains instill this trust without relying on third parties. This trust is by the design of the Blockchains. Let us make it real. Imagine you are selling property. You would involve legal authorities that will validate the purchase, banks that would finance the transaction, and law enforcement agencies that will make sure that bank transfers and ownership transfers happen simultaneously.

Blockchains can overcome this process by writing a smart contract that will lock in the ownership representation from the seller and money from the buyer and swap it without fail each time.

Apart from that, blockchains can be trusted because of the following: 

• Blockchains are highly compatible with other business applications due to their open-source nature.  
• Since it is an immutable data ledger, anything on Blockchain can be proven at any time.  
• It also improves efficiency and speed.  
• It is safe and hack resistant.

One of the most frequently posed Blockchain technology interview questions, be ready for it.

You can think of a block as a digital safe. This safe is capable of storing all forms of data. This can be text, image, video, GIF, or transactional data. Then this block is connected to the other blocks, thus forming a chain called the blockchain.

Blocks can be recognized in a Blockchain using the block height as an identifier.

You see, blocks cannot be defined using a time period because, depending on the number of transactions, each block takes a different time to get validated and added to the chain.

In the previous Blockchain interview questions, we have often quoted a chain that connects different blocks together. Wondering what this chain is all about? 

It turns out that each of these blocks has three types of data points. 

• Hash pointer 

Think of it as a unique identifier of a Block. A fingerprint, for that matter. This number is cryptographically generated, and therefore we call them cryptocurrencies.

Now, this number is also mentioned in the next block to connect one block to another.

So if someone tries to change the hash number of one of the blocks (the hash number changes if the data is changed), the entire chain is changed, and fraud is immediately caught.

• Timestamp 

Timestamp simply puts a date and time on the block. Although Blockchains are self-auditing, this can be important in case someone wants to check transaction sequence externally. 

• List of Transactions

And, of course, there are transactional data in the block. This is what miners would end up validating by solving the complicated mathematical puzzle.

No. Blockchains are immutable forms of data. Any change in the database would lead to a change in the hash string; hence, the entire copy of Blockchain would look different from everyone else's. As a result, there is no merit in trying to change the Blockchain data.

A staple in interview questions for Blockchain, be prepared to answer this one.  

There are two types of records that are available on the blockchain. They can be categorized as follows:

• Transactional Records

This is made up of transactional data showcasing what kind of transactions happened, which wallet addresses, and at what time. This is one of the core purposes of a database.

• Block Records:

Block records include the data pertaining to the information about a block. This can include a timestamp, block number, hash number, etc.

• Nonce

Nonce stands for the 'number used only once. 'It is a 32-bit random number that is used as a base by miners for their hash calculations. Essentially, the validators are trying to calculate a nonce that, when run through a hash function, yields a number that starts with 3 zeroes (in Bitcoin Blockchain) 

Depending on the blockchain, blocks can be of different sizes. As a result, multiple items can be stored on these blocks. These can vary as follows:

• Transactional data of financial information.  
• Identity information of an individual 
• Organizational events (keeping track of the history of key decisions) 
• Management activities (logging in, logging out, etc.) 
• Documentation 
• Records of medical transactions 

Yes. Immutability does not mean you cannot filter out specific data instances. You can easily use a portion of a block or a few blocks within a transaction for your analysis.

This question is a regular feature in Blockchain interview questions for freshers, be ready to tackle it.  

This is one of the blockchain basic interview questions. Depending on the control and nodes, Blockchains can be divided into the following categories:

• Public Blockchain

These blockchains are open for everyone to join. Basically, there are no restrictions when it comes to participants. More so, anyone can see the ledger and take part in the consensus process. For instance, if you wish to deposit cash in a bank, you need to have a bank account with them. Then, you need to visit the bank to deposit it. The bank has the sole discretion of accepting or rejecting your account opening process or deposits/withdrawals.

This is not the case with Blockchains. Most public Blockchains like Bitcoin and Ethereum require no KYC/regulations for one to operate on them. You can buy ether, send it to your friend and even encash it without any subjectivity involved in the process.

• Private Blockchain

Private blockchains are the complete opposite of public blockchains. They are privately run by an individual or an organization that aims to solve trust-related problems within its ecosystem. This Blockchain is completely under the control of the owner and hence is not decentralized.

• Consortium Blockchain

In this Blockchain, the admin gives the rights to individuals to read/write on the Blockchain. Once again, this is not decentralized. 

A ledger is a continuous record of transactions happening in the system. You go to a restaurant, and the staff on the door notes your name, number, and time of entry. That is exactly a ledger. Similarly, if you do a physical bank transaction, an officer makes a note of the details of that transaction. That is, again, a ledger.

Now this ledger can be digital, wherein the entire data is input via keyboard and stored on the local storage or cloud. Or, it could be a physical ledger where registers are used to enter the data using a pen.

Depending on the control, ledgers can be divided into three key categories:

• Centralized ledgers

These ledgers are owned and run by centralized authorities like banks, social media companies, s, and Aadhar, etc. Because these ledgers are under the direct influence of a central authority, they can change or manipulate the data without revealing it to the general public. I am not saying they would, but they can!

• Decentralized Ledgers

These ledgers are not owned by anyone and, therefore, are a safe haven for any form of data. However, storing the data in a decentralized ledger isn't always the best option, especially in cases where trust isn't a concern. For example, if your organization keeps all your employee data in a centralized server, there isn't a problem.

• Distributed Ledgers

The owners of this ledger are spread across different geography, and it is difficult to manipulate this data.  

This could be explained via a simple web2 analogy. The public key is like your email ID. If anyone wishes to share data (or crypto) with you, you can share your public key with them. On the other hand, a private key is like your password. You use it to access the emails and decide which email goes out to whom.

Public keys are meant for identification purposes, while private keys are used to sign transactions with your encryption key.

This is a frequently asked question in Blockchain engineer interview questions.  

Interview questions on Blockchain technology are incomplete without this one. Every aspirant of having a role in Blockchain should know this. The major features of Blockchain are as mentioned below:

• Decentralized

It is not owned by any single central authority. Therefore decisions can't be made by a single person/organization on the data stored on Blockchain. 

• Faster settlement

Blockchains skip the intermediary. As a result, there are a lot of processes that simply cut down, and the settlements are instant. Consider this, when you buy a stock on the market, it usually takes two days for that stock to be credited to your Demat account. With Blockchains, the dream of real-time settlement has come true.  

• Immutability

Blockchains are immutable by nature. Consider it as a database wherein there is no update record function. Depending on the use cases, this could be a really powerful proposition to prove the ownership of something.

• Unanimous

All the nodes or participants on the network agree on the transaction before they are appended onto the blockchain. When a node wants to add a block to the blockchain, it has to gain consensus from at least 51% of the nodes before doing that. Therefore, making blockchains are unanimous in decision-making.  

Blocks are ordered in reverse order within the Blockchain. This means the last block would contain the hash pointer of the previous block and so on. So if at all someone wants to trace the transactions, they can do so by starting from the last block.

BIP and EIP stand for Bitcoin Improvement proposal and Ethereum Improvement Proposal, respectively. Now you cannot upgrade the blockchains by releasing an update on the play store like you would do with web2 applications.

In the case of Blockchains, if an upgrade is rolled out, all the nodes have to agree with it unanimously.

Therefore, an improvement proposal is floated in the community before any upgrade.

Expect to come across this popular question in Blockchain interview questions.  

A blockchain node is a computer connected to the network and participates in verifying transactions. Nodes can be either full nodes, which store a complete copy of the blockchain, or light nodes, which only store a partial copy. 

Moreover, a blockchain defines the requirement for becoming a node. For example, Bitcoin nodes require computational power to solve complicated mathematical puzzles. Similarly, Ethereum requires the user to stake 64 ETH before becoming a node.

A must-know for anyone heading into a Blockchain interview, this question is frequently asked in interview questions on Blockchain.  

A smart contract is a self-executing contract with the terms of the agreement between buyer and seller being directly written into lines of code. The code and the agreements contained therein are stored on the blockchain.

One may argue that this is not very different from a traditional code. But then the immutability of it makes it superior. This means code is the law in the Blockchain regime. If a condition is defined, it will execute, without subjectivity, when it is met.

A hash function is a mathematical function that takes an input of any size and produces an output of a fixed size. In the context of blockchain, hash functions are used to create a unique code, or "hash," for each block in the chain.

Think of a hash function as a black box. This box takes input from one end (can be a number, image, video, etc.) and gives an alphanumeric output from the other side. The output cannot be traced back to the input in any way, but the same input will always generate the same output.